The present invention relates to mechanical seismic restraint means which are used in nuclear safety systems. In particular, the seismic restraint means of the present invention are usable with generally cylindrical nuclear radiation detectors which must be mechanically supported in a vertical position about a nuclear reactor vessel, and which detectors must be substantially isolated from vibrational force resulting from seismic activity.
The typical out-of-core radiation detector utilized in nuclear safety systems is an ion chamber of substantial length, for example about 6 to 12 feet long. The ion chamber typically utilizes concentric cylindrical electrodes which are maintained a fixed distance apart, with opposed electrical potentials for attracting respectively the opposed charge particles which are generated in the ion chamber by neutrons from the reactor core. Such ion chambers are thus used to monitor reactor activity and to indicate the operational status of the reactor. The ion chamber is typically mounted in an elongated tubular thimble which is typically open ended at the top and may be closed or open ended at the bottom. A plurality of such thimbles are spaced around the reactor vessel in a predetermined array to permit sampling of the neutron flux level in the vicinity of the reactor vessel. Seismic activity can produce significant vibrational forces on the elongated ion chambers and give rise to electrical noise in the output signal from such ion chambers. This electrical noise is thought to be a result of the vibratory motion of the electrodes relative to each other in the ion chamber.
The tubular thimbles within the ion chamber radiation detectors are typically mounted can have a variable inside diameter, as is typical for commercially available piping which is used in forming the reactor thimbles. A typical 6 inch nominal diameter schedule 80 piping typically will have an inside diameter which ranges from about 5.931 inch to 5.730 inch, and a 6 inch nominal diameter schedule 40 pipe has a resultant inside diameter range of from about 6.197 inch to 6.034 inch. With such inside diameter range variations of up to about 0.2 inch inside diameter, it is difficult to provide a detector support means which provides a uniform fit and support of the detector assembly for the wide range of thimble inside diameters. It has thus been necessary to provide a rather loose fit in current detector support assemblies relative to the thimble ID. This leads to excessive motion of the detector assembly during seismic activity and results in undesirable electrical signals generated by motion of the internal detector parts. The presently utilized detector support assemblies are also rigid assemblies and there is thus no damping of impact shocks experienced by the assembly during seismic activity.
It is the object of the present invention to provide a seismic restraint means which will provide a uniform fit for a detector assembly within a tubular thimble where the range of thimble inside diameters has significant variation. It is also desirable to provide a constant restraining force between the detector and the thimble with the seismic means capable of damping the vibrational modes which are induced during seismic activity.